Stage of development affects penetration of calcium chloride into apple fruits

Using young fruits or fruit segments, rates of penetration of ⁴⁵CaCl₂ across apple fruit cuticles (Malus x domestica) were studied during fruit development in two consecutive years (’?Golden Delicious’ in 2000, ’?Cox Orange Pippin’ and ’?Boskoop’ in 2001). The aqueous donor solutions contained 5 g l^—1 CaCl₂ and 0.2 g l^—1 Glucopon 215 CSUP as wetter. A droplet (5 μl) was applied to each fruit disc and allowed to dry within 1 h. Amounts penetrated were estimated immediately after droplet drying (0 h) and after 6 and 24 h penetration, respectively. Temperature during penetration was 20 ?C and 100 % humidity was maintained throughout the experiment, once the droplet had dried. Rates of penetration were greatly affected by the stage of fruit development. With all varieties tested, highest rates were measured during the early stages of fruit development (before June drop) when approx. 100 % of the CaCl₂ applied penetrated within 24 h. After June drop, when trichomes had vanished and most stomata developed into lenticels, penetration rates decreased rapidly and large variability among the samples developed. This pattern was similar with all three varieties. The involvement of trichomes, stomata, and lenticels as preferential sites of penetration of calcium chloride across the apple fruit cuticles is discussed. Using a model calculation, it is argued that ten or more spray applications are needed to significantly increase calcium contents of apples. This has two causes: Only a small fraction of the spray liquid is intercepted by the fruits, and penetration can be rather slow.